The object-relational impedance mismatch is a set of conceptual and technical difficulties which are often encountered when a relational database management system is being used by a program written in an object-oriented programming language or style; particularly when objects and/or class definitions are mapped in a straightforward way to database tables and/or relational schemata. This practice has been recommended and documented by some object-oriented literature as a way to use databases in object-oriented programs.
The term Object-relational impedance mismatch is derived (etymologically) from the electrical engineering term impedance mismatch.
Reference:
http://en.wikipedia.org/wiki/Object-Relational_impedance_mismatch
